1. Field of Invention
This invention relates in general to subsea wellheads for use in oil and gas wells, and in particular to a shoulder ring that fits on a casing hanger to provide support for a lock-down ring.
2. Description of Prior Art
A subsea well that is capable of producing oil gas typically has an outer or conductor housing secured to a string of conductor pipe which extends some short depth into the well. An inner or high pressure wellhead housing lands in the conductor housing and secures to an outer or first string of casing, which extends coaxially through the conductor to a deeper depth into the well. Depending on the particular conditions of the geological strata above the target zone (typically, either an oil or gas producing zone or a fluid injection zone), one or more additional casing strings will generally extend through the outer string of casing to increasing depths in the well until the well is cased to the final depth. Each string of casing is supported at the upper end by a casing hanger, which usually lands in and is supported by the wellhead housing.
Where multiple casing hangers are landed in the wellhead housing, they are generally stacked on one another in the wellhead housing. The lowest string of casing extends into the well to the final depth, this being the production casing. The strings of casing between the outer casing and the production casing are usually referred to as intermediate casing strings.
Between each casing hanger and the wellhead housing, a casing hanger packoff or annular seal assembly is set to isolate each annular space between strings of casing. The weight of the casing hanger and the casing hanging from the casing hanger can prevent upward movement of the casing hanger under some circumstances. A lockdown mechanism, however, is required to lock the casing hanger in place and can be used to lock the annular seal assembly to the wellhead housing when the casing hanger is subjected to high pressures. Those high pressures can cause the casing hanger to move axially upward. Expansion and contraction of the casing can also cause the annular seal to be compromised and cause leaking.
The lockdown mechanism can be a separate assembly lowered onto the upper casing hanger and locked to the wellhead housing. The lockdown mechanism can alternately comprise a lockdown ring carried by the casing hanger and moved radially to engage a profile in the wellhead housing. The radial movement may be in response to setting of the casing hanger seal. Engaging surfaces mate to resist the lockdown forces. In some current designs, the capacity of the lockdown ring is limited by the yield strength of the material used to form the casing hanger and the wellhead housing. Also, debris can clog the area and inhibit movement of the lockdown mechanism to the expanded position.